I do simulating natural convection of the air within the enclosure model. I considered "Boussinesq approximation" for the density term of the air and the results seems to be OK. but when I do change to "Ideal gas" I receiving "disconvecgence error of Temperature". transient analysis is employed. temperature difference between minimum and maximum imposed to the model is about 18C.

What is the reason of getting such an error?

In fact, I am validating the results of experimental work and I have two similar cases in which one with melting/solidification model and one without melting/solidification model.
In the case without melting/solidification model,the results are converged in the "ideal gas" mode for the density of the Air. but in the case with melting/solidification model, the results are converged when "Boussinesq approximation" is deployed.